I'm want to get a better graphic card to run along side my 6900, wondering which would card would be the best. I was thinking about getting the Nividia GTX 780
Generally speaking, nVidia and AMD don't work well together, so a 780 and a 6900 series are not a good mix. If your motherboard supports Crossfire you can find the same 6900 series GPU and run them in tandem (of course, there is the frame-rating issue, but I believe AMD will have this resolved soon).
If that doesn't sound good, try nVidia's 760, or AMD's 7970 for a single card solution.
Hope this helps.